1. Field of the Invention
The present invention relates to cutting tools of the type utilizing fixed cutter cutting elements and, specifically, to an improved cutting element retention system for mounting a fixed cutter cutting element on the face of an earth boring drill bit.
2. Description of the Prior Art
"Fixed cutter" is the industry standard terminology used to describe a type of bit which does not use a cutting structure with moving parts, such as a rolling cone bit. Fixed cutter terminology has been officially adopted by the International Association of Drilling Contractors (IADC) Drill Bit Subcommittee. Fixed cutter categorization includes Polycrystalline Diamond Compact (PDC), Thermally Stable Polycrystalline (TSP), Natural Diamond, and Other. "Other" is a category of fixed cutter bit that does not use a diamond material of any kind as its cutting element. Commonly tungsten carbide is used as the substitute. It is understood that throughout the following discussion, where reference is made to "diamond" cutting elements, PDC, TSP, Natural diamond and "Other" cutting elements such as tungsten carbide are also intended to be included.
A variety of cutting tools are known which are well suited for receiving diamond cutting elements as the cutting or wear portion of the tool. Known applications include, for instance, the mining, construction, and oil and gas exploration and production industries. An important category of such tools is the earth boring drill bit of the type used to drill oil and gas wells. The commercially available earth boring bits are generally classified as either rolling cutter bits, having steel teeth or tungsten carbide inserts, and diamond bits which utilize either natural diamonds or artificial or man-made diamonds.
The diamond earth boring bits are further classified as either steel bodied bits or matrix bits. The steel bodied bits are machined from a steel block and typically have cutting elements which are press-fit into openings provided in the bit face. The matrix bit is formed by coating a hollow tubular steel mandrel in a casting mold with metal bonded hard material, such as tungsten carbide. The cast mold is of a configuration which will give a bit of the desired form. The cutting elements are typically either polycrystalline diamond compact cutters braised within an opening provided in the matrix backing or are thermally stable polycrystalline diamond cutters which are cast within recesses provided in the matrix backing.
The prior art cutting element retention systems were therefore basically of two styles: (1) a tungsten carbide stud press-fit into a recess on the bit face; and (2) mechanical and/or braised attachment of the cutting element into a cast recess (hole) provided in a tungsten carbide matrix bit face or attachment of the cutting element into a pocket (boss) provided on the cast matrix bit face. In either system, the forward portion of each system was designed to provide sufficient cutting element attachment and retention. The rearward portion of the system behind the cutting element was required to provide mechanical strength sufficient to withstand the forces exerted during the drilling operation. An essential requirement was that the rearward portion of the retention system not unduly flex, break or erode.
The stud carrier typically utilized in the above applications came in various styles including "flat top" and "round top" styles. Whatever style, the stud carrier typically featured a solid, blocky mass behind the cutting element without the presence of any void areas. Likewise, in the mechanical and/or braised attachment system, a solid, blocky mass of cast tungsten carbide was utilized behind the cutting element to provide sufficient mechanical strength. This mass typically did not feature void space and was approximately the shape of a sphere quarter-section, positioned with one flat side against the back of the cutting element and another flat side toward the bit face, causing the rounded edge to become the exposed top of the rear of the pocket mass.
In either system, three non-optimum conditions were required to be tolerated. First, as the cutting element wore, the bearing area of the bit face on the bottom of the borehole being drilled substantially increased. This caused an increasing amount of heat to be created which then was conducted through the cutter retention system. Excessive heat is detrimental to the heat sensitive cutting elements.
Secondly, the progressively increasing wear flat area created as the cutting element wears decreased product performance to the point of causing termination of the bit run due to excessive torque, excessive bit weight requirements, poor penetration rate and/or poor cutter retention rate.
Thirdly, because of the wear characteristics and associated limitations of the prior art cutter retention systems, currently used bits are frequently returned from the field with greater than 50% of the original diamond material remaining on the bit face
The present invention has as its object to provide a drill bit with an improved cutting element retention system whereby a more complete cutting element usage is obtained.
Another object of the invention is to provide a cutting element retention system which provides sufficient strength for the cutting element while presenting an essentially minimum surface area to drag on the bottom of the borehole being drilled as the cutting element wears. Considerable forethought has been given to maintaining structural strength integrity in the retention system while maximizing the percent of diamond cutting element depth usable.
Another object of the invention is to reduce the cutting element temperature by generating less heat during use, then by quickly removing any unwanted heat which still is generated via convection heat transfer from the cutting element retention system.